The present invention relates generally to the field of tankless fluid heaters. These are flow-through devices for the instantaneous heating of a fluid by passing the fluid through a chamber containing a heating element. Several versions of such an apparatus have been particularly adapted to heating water, with the objective of serving the function ordinarily performed by a standard tank-type water heater.
Tankless water heaters in general have an advantage over tank-style water heaters in that they apply energy to heat only water about to be used, rather than continually heat and reheat a stored reservoir of water. A principal challenge in tankless water heater design is that widely varying flow rate demands are present in a typical use, and ideally a constant set water temperature at the output will be available regardless of flow velocity or flow volume. Further, the bounds on flow demand in, for example, typical residences may vary widely by size of residence or size of family.
The latter problem has been addressed by modular tankless heater design, whereby one or more heating elements may be placed in contact with the moving water, according to the expected maximum flow to be serviced. The multiple elements may be in a single water chamber or in a set of chambers connected in series between cold water input and heated water output. For all such designs, it is advisable to maximize the transfer of heat to the flowing liquid by moving the liquid sequentially across the heating elements. Most inventions of this sort have disclosed a feedback mechanism, either analog, digital or microprocessor-based, to regulate the output temperature by turning off the elements when the water gets hot enough, and a flow sensor to assure that the elements operate when fluid is moving through the system, shutting off when the flow ceases.
For example, U.S. Pat. No. 5,408,578 to Bolivar discloses a tankless heater with a plurality of elongated chambers, each of which contains a heating element, water under pressure enters the first heating chamber at the bottom and fills it. A pair of ports of different sizes connect the first heating chamber with an adjacent heating chamber. The size difference allows better distribution of water to the heating elements. The design also includes an entrance chamber containing a flow control switch that activates the heating circuitry when water moves through the chamber. Hurko, U.S. Pat. No. 4,808,793 discloses a tankless electric water heater which includes an open ended folded tubular conduit having a separate metal-sheathed emersion heating element inserted into each end of the conduit. It also includes a self-regulatory heating cable, either in or wrapped around the tubular conduit, that is energized independently of the main heating elements and keeps the standing water in the chamber at a set temperature. Davidson, U.S. Pat. No. 4,604,515, discloses a chamber housing that is divided into a plurality of equal subchambers by barrier walls, with each subchamber having a heating element responsive to a separate temperature sensor.
White, U.S. Pat. No. 5,479,558, discloses a compact tankless water hater in which a single water chamber, filled from the bottom, contains four individually controlled heating elements. A pressure responsive flow switch activates circuitry which sequentially energizes the heating elements according to need. Posen, U.S. Pat. No. 5,438,642 discloses a serpentine chamber for water flow, carrying the water sequentially along in a plurality of heating elements, which can be either flat plate elements that constitute combination heating and chamber partition assemblies. Fernandez, U.S. Pat. No. 5,325,822, discloses modular units having two connected chambers, each with a heating element, that may be connected in series. Temperature sensors in the first and second chambers of each module provide signal inputs to energize each heating element of each chamber for a period of time proportional to the temperature difference between the first sensor and the desired set temperature.
The present invention comprises a compact tankless water heater capable of configuration to accommodate a range of potential demands. A rectangular heat transfer chamber is divided by a central rib wall into two subchambers. An inlet opening at the bottom of the apparatus is centered on the rib wall so that water enters and fills both subchambers simultaneously. A plurality of heating elements is mounted in the heat transfer chamber, with the preferred design capable of fitting from one to four elements, depending upon the expected demand for hot water. Thus, the same configuration may be installed whether demand requires four elements (typically a house for a family of four), or just one (such as an individual sink).
A notch passageway at the top of the central rib permits water to flow between the subehambers if one fills faster than the other. An exit chamber adjacent to the heat transfer chamber is connected at the bottom to the plumbing in the facility being serviced. A notch passageway at the top of the heat chamber wall allows water to flow across and down into the exit chamber.
A flow sensor measures the rate of water movement, and temperature sensors are placed at the water inlet, the outlet, and near the tops of the two heating chambers. With flow rate, incoming temperature and outgoing temperature as inputs, a microprocessor based controller regulates the energy to the heating elements to maintain a set point water temperature. Safety of the unit is enhanced by a mechanical thermal cut-off switch as well as protective relays that open when an over-temperature condition in the chamber is sensed.